Giant clams evolved over 65 million years
ago in the Eocene, along with modern corals. Both clams and corals lead symbiotic lives
along with millions of single cell dinoflagellate algae called 'zooxanthellae'.

Adult clams can obtain over 90% of their food requirements from the photosynthetic
products of the algae, but when particulate organic matter (POM) is present an adult clam
can obtain about 33% of its carbon requirements from POM. The third source of nutrition is
dissolved organic matter (DOM) but it is probably more important in larval and post-larval
stages than in sub-adult and adult clams. Clams also act as natural biofilters as they
take up dissolved ammonia and nitrate from the surrounding seawater to supply their
symbiotic zooxanthellae with nitrogen for growth.

Maybe in the near future giant clams will become accepted by aquarium enthusiasts as an
ideal living biofilter. Someday giant clams may be found in aquaculture tanks in a Moon
Colony to supply food, excess oxygen in sunlight, and as biofilters for fish and other
aquacultured animals.

Until then, the aquaculture of giant clams for food and restocking of overharvested reefs
is an example to follow for other endangered species.

Tridacna
gigas, the true giant clam. It can reach to > 1.3m in shell length and
hundreds of kilograms in weight, living to >50 years of age. This is the
largest species of bivalve mollusc to have ever lived in the fossil record of our
planet. It was being driven to extinction in some parts of its natural distribution
(SE Asia, Australia, Micronesia) by poachers in the 1970s. Natural high density
populations are still present on the Great Barrier Reef of Australia. This species
is on the Threatened Species list of IUCN and CITES.

Tridacna
derasa, the smooth giant clam. Second largest species reaching > 55 cm shell
length. On the Great Barrier Reef (GBR) it is found only on mid-shelf and
outer-shelf reefs where turbidity is lower than inner-shelf reefs. The background
pattern is of juvenile T. derasa reared by Dr. Braley at the Barrang Lompo Island
giant clam hatchery, So. Sulawesi, Indonesia in the mid-90s. The distribution is SE
Asia, Micronesia to Tonga and Australia.

Tridacna
squamosa, the scaly or fluted giant clam held by Dr. Rick Braley at the Aitutaki
atoll giant clam hatchery, Cook Islands, early 90s. This species reaches 40 cm shell
length and has light to moderate byssal attachment as adults. It is widespread from
E. Africa & Red Sea to French Polynesia.

Tridacna
maxima, the elongated, rugose, or small giant clam. This species grows up to 40
cm shell length and has great byssal attachment to the substrate. The distribution
is widespread and similar to T. squamosa. The mantle is very colourful and
the clam is sought after in the aquarium trade.

Tridacna
crocea, the boring or crocus clam. This is the smallest species of giant clam
reaching to 15 cm shell length. It has a wide byssal opening at the shell base and
therefore burrows into the substrate so that only the extended colourful mantle touches
the substrate surface. Like T. maxima, it is a good species for the
aquarium trade. The distribution is SE Asia, Australia and western Pacific.
In places on the GBR densities can be >100/ sq m.

Now considered Tridacna mbalavuana [exTridacna
tevoroa Lucas, Ledua & Braley, 1990], the tevoro or 'devil' in
Fiji language) clam. This quite large (>50 cm shell length) clam is
found in moderately deep water (14-35m depth). The species
is unusual in that it is the only species of the genus Tridacna which
does not have the mantle overhang the edge of the shell. It was first
cultured at the Tonga Fisheries hatchery at Sopu, Tongatapu around 1990-91.
There have been reports of findings off outer reefs of the GBR, so the natural
distribution may be further extended.

Hippopus hippopus, the
horse's hoof clam, the bear paw clam, the rolling clam, or the strawberry clam. This
species reaches 50 cm shell length and is often found on sandy areas and on seagrass beds.
The mantle does not extend over the shell edge. It is widespread in SE Asia,
Australia to Vanuatu and parts of Micronesia. It was recently extinct from Fiji and
Tonga.

Hippopus
porcellanus, the china or porcelain clam. This species has a smoother shell
than H. hippopus and generally reaches <50 cm shell length. The natural
distribution of the species is eastern Indonesia, southern Philipines, Palau, and Papua
New Guinea. It has been cultured in Palau, Philippines and Indonesia, but continues
to be a rare species. Dr. Braley took this photo of broodstock collected from
reefs around Barrang Lompo Island, So. Sulawesi, Indonesia in the mid-90s.

This
is a possible 9th species of giant clam which was named by Sirenko and Scarlato(1991)
based on a shell specimen only. The authors have called this clam Tridacna
rosewateri. It is from the Indian Ocean (Mauritius area). It
is said to be similar to T. squamosa, as the scutes in this picture
show.

Tridacna
costata sp., nov. (2008)
[found in the Red Sea and described by Richter et al., Collapse of a New Living
Species of Giant Clam in the Red Sea, Current Biology
(2008), doi:10.1016/j.cub.2008.07.060 ]. This is a 10th species but there is some contention about it being a new species.

There is a newly resurected species very similar to T. maxima and has recently (2014) had genetic traits to distiguish it from T. maxima. It was named Tridacna noae (Roding 1798). Researchers recently studied specimens from Ningaloo reef, Western Australia. This clam species had been generally called the teardrop maxima by aquarium enthusiasts because of its unique pattern of the mantle. Dr. Braley has worked in 2016 with Prof. Paul Southgate and PhD student Thane Militz on embryology of this species and other research at the Papua New Guinea National Fisheries Authority at the Nago Island aquaculture facility near Kavieng, New Ireland.

To see giant
clam culture video clips on YouTube taken by Gerald Heslinga (currently of Indo-Pacific
Seafarms, Kona, Hawaii) in the late 1980s at Orpheus Island, North Queensland,
Australia and also in Palau in the mid-1990s click on the urls below:

Ocean intertidal growout of Tridacna gigas at
Orpheus Island, ~1989. After 3 years of age the clams can be left on the substrate
without protective cages or netting. Research on production rates indicated 29
tonnes/ha/yr could be produced by these 'Solar Animals'.

Translocations of Giant Clams

OPERATION
CLAMSAVER (named by the Great Barrier Reef Marine Park Authority [GBRMPA]: Movement
of cultured T. gigas from Orpheus Island Research Station (OIRS) reef flat to
reefs in the GBR with the Australian Naval Vessel M.V. Tarakan. Dr. Rick
Braley was consulting for James Cook University to oversee the translocations, while the
GBRMPA had an assisting officer / observer aboard. When the operation first began
there was a big media coverage. All the major television channels had interviewers
and camera crew fly in by float plane to get this on the national news. The plan
worked out by GBRMPA, JCU, and AIMS [Australian Institute of Marine Science] was to
move clams to reefs which were considered Source Reefs. Currents travelling
past these reefs during austral summer [same time as clam spawning season] theoretically
would carry the larvae resulting from spawning on the source reef to other reefs
designated Sink Reefs. Given that we attempted to move clams from a single
cohort to each source reef, it may be possible in future to confirm the Source / Sink
theory by finding juvenile clams on sink reefs which would be genetically identical to the
artificially-placed cultured populations translocated to these source reefs from
OIRS. Without giving exact details of reef names, the details of numbers moved,
dates, and general area are shown below [note: the trips which Dr. Braley was
present at, representing JCU show an asterisk *]:

Date

General Location

Number of clams translocated

21 May 1992*

Reefs in the vicinity of Townsville

1,305

23 May 1992*

Reefs in the vicinity of Townsville

1,354

29 May 1992*

Reefs in the vicinity of Cairns

1,020

30 March 1993

Reefs in the vicinity of Innisfail

1.400

1 April 1993

Reefs in the vicinity of Innisfail

1,166

There
were also translocations of one cohort of T. gigas made by the Great Barrier Reef
Aquarium (now called Reef HQ) from OIRS to 7 (seven) reefs in the vicinity of
Townsville. Each reef had 60 clams placed there (total 420 clams).

Only
in 2003 has there been a resurgence of interest by JCU and other organisations
in these translocated clams. Some genetic work has recently been done
on the now-grown-up clams by JCU. Aquasearch would like to have some future
involvement with any studies which are carried out on these high density
artificially placed populations.

Photo of Rick Braley holding up a Tridacna gigas at Orpheus Island, in an action photo of water squirting from both siphons. This photo and a story entitled "Giant clams like cabbages in an ocean garden" written by Sybil Nolan appeared on the front page of the Weekend Australian 9-10 May 1987 (below):

I was working as a Smithsonian / Peace Corps Environmental program volunteer with Fisheries, Tonga. I was on a turtle nesting survey and seabird survey in uninhabited islands of the southern Ha’apai group of Tonga. The photos below are of 4 islands in this group with Tonumea being the northernmost. Note the two channels cutting out through the fringing reef on the southern side of Tonumea. This is where I saw my first true giant clam, Tridacna gigas. Joseph Rosewater in Indo-Pacific Mollusca, vol. 1, no.6 (April 30, 1965) does not show the distribution going eastward beyond Fiji. While I do not have a photo of that clam it is indelibly printed on my mind. I was used to diving in Tonga to collect T. derasa, T. squamosa,or T. maxima but the size of this clam took my breath away. It was humbling to look at how massive a bivalve mollusc could grow. As it turns out, this uninhabited island was possibly one of the last refugias for this species in Tonga. Selective collection for food by the Polynesian people was the cause of it local extinction.

Close-up view of channels on the SSW side of Tonumea is. where I saw my first Tridacna gigas.

Snorkel trails, Magnetic Island

Giant clams moved into the snorkel trails on Magnetic Island which Dr. Braley designed and setup for TOBMI (Tourism Operators and Businesses Magnetic Island). The move was made on 13 June 2013 and the following link shows the evening news on Channel 7 (Townsville) on 19 June 6pm 2013: